Once we ask the backend to close the connection after a given request
there is no benefit from putting the backend connection back in the
pool. It's actually a surefire way to force a subsequent backend fetch
to fail its first attempt and go straight to its extra chance.

Since we try to maximize connection reuse this would have to come from
VCL and a user asking for the backend to close the connection should
have a good reason to do so, for example when the backend is known to
misbehave under certain circumstances.

Closes #3400
Refs #3405